1. Field of the Invention
The present invention relates to a process for developing a latent electrostatic image on an electrophotographic copying material by means of a toner dispersion that is applied to the copying material at the beginning of the development process. The toner is precipitated according to the surface charge of the copying material, and, at the end of the development process, the unused toner dispersion is squeegeed off the copying material. The invention also relates to an apparatus for carrying out the foregoing process.
2. Description of the Prior Art
German Offenlegungsschriften Nos. 1,597,820 and 2,331,253 disclose development apparatus for electrophotographic copying materials in which the material provided with a charge image is guided through a development zone wherein a toner dispersion is applied to that surface of the copying material which carries the electrostatic charge image. The copying material passes through a pair of infeed rollers, the upper one of which is uniformly wetted with the toner dispersion. Thus the upper infeed roller applies a uniform film of toner dispersion to the copying material. At the end of the development zone there is a pair of squeegee rollers which squeegee the surplus toner dispersion off the surface of the copying material.
These development apparatus have the disadvantage that there is no steady toner supply at the upper infeed roller under all conditions and that the uniformity of toner application to larger areas is unsatisfactory. In order to obtain a more complete and more uniform development of the electrostatic charge image by the simplest means possible, an apparatus has been developed in which there is provided, above the web of copying material and between a pair of infeed rollers and a pair of squeegee rollers, a distribution roller that has a surface composed of soft and porous material and is grounded. Such apparatus is disclosed by German Utility Pat. No. 7,418,392. It has been found, however, that through use of such apparatus white lines in large black areas of the latent electrostatic charge image can only be insufficiently reproduced during development. In most cases the thin white lines in the black area are covered with toner, i.e., these lines either cannot be recognized at all or only insufficiently so that the contrasting effect of the powder image is imperfect. Another disadvantage is that the toner attracted by the surface of the distribution roller adheres to it and cannot be used for the further development process.